Apparatus for imaging the anatomy

ABSTRACT

The present invention pertains to a fiducial implant for the human body that is detectable by an imaging system. The invention is comprised of a first portion and a second portion. The first portion is configured to be detected by an imaging system when palce beneath the skin. The second portion is configured for fixed attachment to a bone beneath the skin without penetrating entirely through the bone and without fracturing the bone. The first portion is sufficiently large and comprised of a material for detection by an imagaging system, and sufficiently small to avoid the distortion of the skin when placed at an interface between the skin and the bone. The first portion also has at least a portion which is spherical and defines a surface for cooperating with a tool for securing the second portion to the bone. Additionally, the placement of three fiducial implants into a portion of anatomy of the human body allows for the recreation of a particular image slice of the portion of the anatomy taken by an imaging system with respect to a first time period, at subsequent imaging sessions and also with different scan modalities. This provides a doctor with the ability to accurately follow the progress of the portion of the anatomy of interest. Moveover, the existence of three fiducial implants allows a target to be identified within the portion of anatomy relative to an external coordinate system. The portion of anatomy with the target may then be operated on, for instance, robotically, or precisely irradiated.

This is a division of application Ser. No. 119,353, filed Nov. 10, 1987,now U.S. Pat. No. 4,991,579.

BACKGROUND AND DISCUSSION OF THE INVENTION

Diagnostic techniques that allow the practicing clinician to obtain highfidelity views of the anatomical structure of a human body have provedhelpful to both the patient and the doctor. Imaging systems providingcross-sectional views such as computed tomographic (CT) x-ray imagers ornuclear magnetic resonance (NMR) machines have provided the ability toimprove visualization of the anatomical structure of the human bodywithout surgery or other invasive techniques. The patient can besubjected to scanning techniques of such imaging systems, and thepatient's anatomical structure can be reproduced in a form forevaluation by a trained doctor.

The doctor sufficiently experienced in these techniques can evaluate theimages of the patient's anatomy and determine if there are anyabnormalities present. An abnormality in the form of a tumor appears onthe image as a shape that has a discernable contrast with thesurrounding area. The difference in contrast is due to the tumor havingdifferent imaging properties than the surrounding body tissue. Moreover,the contrasting shape that represents the tumor appears at a location onthe image where such a shape would not normally appear with regard to asimilar image of a healthy person.

Once a tumor has been identified, several methods of treatment areutilized to remove or destroy the tumor including chemotherapy,radiation therapy and surgery. When chemotherapy is chosen drugs areintroduced into the patient's body to destroy the tumor. During thecourse of treatment, imagers are commonly used to follow the progress oftreatment by subjecting the patient to periodic scans and comparing theimages taken over the course of the treatment to ascertain any changesin the tumor configurations.

In radiation therapy, the images of the tumor generated by the imagerare used by a radiologist to adjust the irradiating device and to directradiation solely at the tumor while minimizing or eliminating adverseeffects to surrounding healthy tissue. During the course of theradiation treatment, the imaging system is also used to follow theprogress of the patient in the same manner described above with respectto chemotherapy.

When surgery is sued to remove a tumor, the images of the tumor in thepatient can guide the surgeon during the operation. By reviewing theimages prior to surgery, the surgeon can decide the best strategy forreaching and excising the tumor. After surgery has been performed,further scanning is utilized to evaluate the success of the surgery andthe subsequent progress of the patient.

A problem associated with the scanning techniques mentioned above is theinability to select and compare accurately the cross section of the sameanatomical area in images that have been obtained by imagers atdifferent times or by images obtained essentially at the same time usingdifferent image modalities, e.g., CT and MRI. The inaccuracy in imagecomparison can be better appreciated from an explanation of the scanningtechniques and how the imaging systems generate the individual imageslices within a cross-sectional "slice" of the patient's anatomy. Aslice depicts elemental volumes within the cross-section of thepatient's anatomy that is exposed or excited by a radiation beam or amagnetic field and the information is recorded on a film or othertangible medium. Since the images are created from slices defined by therelative position of the patient with respect to the imager, a change ofimage slices of the orientation of the patient results in differentelemental volumes being introduced into the slice. Thus, for comparisonpurposes two sets of approximately the same anatomical mass taken atdifferent times, do not provide comparable information that can beaccurately used to determine the changes that occurred between two imageslices in the sets, since it is unknown to what extent the twoindividual image slices selected from the respective sets depictidentical views.

The adverse effects on the medical practice of such errors isexemplified by diagnostic techniques utilized by the surgeon or othersin diagnosing a tumor within a patient. If a patient has a tumor, itssize density and location can be determined with the help of imagesgenerated by a scanning device. For the clinician to make an assessmentof the patient's treatment, two scanning examinations are required. Thepatient is subjected to an initial scan that generates a number ofslices through the portion of the anatomy, for instance the brain, to bediagnosed. During scanning, the patient is held in a substantially fixedposition with respect to the imager. Each slice of a particular scan istaken at a predetermined distance from the previous slice and parallelthereto. Using the images of the slices, the doctor can evaluate thetumor. If, however, the doctor wants to assess changes in theconfiguration of the tumor over a given period of time, a second or"follow-up" scan has to be taken.

The scanning procedure is repeated, but since the patient may be in aposition different from that in the original scan, comparison of thescans is hampered. Slices obtained at the follow-up examination may beinadvertently taken at an angle when compared to the original slices.Accordingly the image created may depict a larger volume than that whichwas actually depicted before. Consequently, the surgeon may get a falseimpression of the size of the tumor when comparing scans taken atdifferent periods. Because of this, slice-by-slice comparison cannot beperformed satisfactorily.

Similarly for certain surgical techniques it is desirable to haveaccurate and reliable periodic scans of identical segments of the tumorwithin the cranial cavity. If the scans before and after surgery areinaccurate, the doctor may not get the correct picture of the result ofsurgery. These same inaccuracies apply to other treatments such aschemotherapy discussed above.

Additionally, with regard to imaging systems and the integral part theyplay in surgical and other tumor treatment procedures, there is a dearthof methods currently existing that allow a determination of a desiredlocation within the body at a given time. For example, U.S. Pat. No.4,583,538 to Onik, et. al. discloses a localization device that isplaced on a patient's skin which can be identified in a slice of a CTscan. A reference point is chosen from a position on the device whichexactly correlates to a point on the CT scan. Measurements of thelocalization device on the CT scan is then correlated to the device onthe patient.

Exterior devices have been utilized in an attempt to solve some of theseproblems with accuracy such as that shown in U.S. Pat. No. 4,341,220 toPerry which discloses a frame that fits over the skull of a patient. Theframe has three plates, each defining a plurality of slots on three offour sides. The slots are of varying lengths and are sequentiallyordered with respect to length. Frame coordinates defined and found onthe frame correspond to the varying heights of the slots. When slices ofthe skull and brain are taken by an imaging device, the plane formed bythe slice intersects the three plates. The number of full slots in theslice are counted with respect to each plate to determine the coordinateof a target site with the brain. Accordingly, only one CT scan is neededto pinpoint the coordinates of the target.

Other attempts have included the use of catheters for insertion into theanatomy. For example, U.S. Pat. No. 4,572,198 to Codington discloses acatheter with a coil winding in its tip to excite or weaken the magneticfield. The weak magnetic field is detectable by an NMR device thuspinpointing the location of the catheter tip with respect to the NMRdevice.

Applicant's invention largely overcomes many of the deficiencies notedabove with regard to imagers used heretofore. The invention relates to amethod and apparatus for insuring that scans taken at different timesproduce images substantially identical to those of previous scans evenif they are from different image modalities at different times. Thisinsures that a more accurate assessment of any changes in anatomy isobtained. As a result, the doctor can be more certain as to the size,location and density of the tumor, or a section thereof, that is locatedin the cranial cavity.

This ability will enhance the use of surgical techniques in removing orotherwise eliminating the tumor in particular by those noninvasivetechniques such as laser technology. By having the ability to defineaccurately the tumor location and size, laser beams can be focuseddirectly on the tumor. Intermittently, as part of surgical techniques,scans can be made to determine if the tumor has moved or substantiallychanged in size as a result of the surgery. The laser or other surgicalinstrument can be adjusted accordingly. Because of the accuracy of theimaging techniques produced by the invention, the doctor can beconfident that the amount of healthy tissue destroyed during surgery isminimized.

A method adopted by the invention disclosed herein utilizes fiducialimplants or implants to define a plane which cooperates with the imager,or other computer, and particularly the data processing capabilities ofthe imager to insure that subsequent scanning results in slicessubstantially parallel to those taken during the initial scan. Thefiducial implants are implanted beneath the skin into the calvania andare spaced sufficiently from one another to define a plane. The patientwith these implants implanted is placed in the scanning device in theconventional manner and scanned to provide the images of consecutiveparallel slices of a given thickness along a predetermined path throughthe cranial cavity.

As the scans are taken, one or more slices will be needed to accommodatepart or all of each fiducial implant. The computational features of theimager or other computer will take into account the spatial relationshipbetween any selected plane of a slice and that plane defined by thefiducial implants. Because of this capability, images taken insubsequent scans at different points in time, at different angles can bereconstructed to be substantially identical with the slices takenoriginally.

Fiducial implants for this purpose are specially configured and made ofmaterial that enables their implantation into the skull and the abilityto be detected by scanning devices. The fiducial implant as disclosedherein is configured to insure that during implantation it does not haveadverse effects on the skull such as cracking or extending through tothe cranial cavity. Nor is it sufficiently exposed between the skull andthe skin to distort any external features of the anatomy. Furthermore,the fiducial implant is positioned at least on a portion of the skull atthe interface of the skin and the bone of the skull to facilitate itsimaging by the imager. At least a portion of the implant is symmetricalin cross-section such that slices taken of the cranial cavity forexample can be used to locate the center of mass of the implant. Thisinsures accuracy in using the implant image as a reference point totransform the subsequent slices of the follow-up examination into theproper position and orientation.

The above has been a description of certain deficiencies in the priorart and advantages of the invention. Other advantages may be perceivedfrom the detailed description of the preferred embodiment which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the present invention and many of theattendant advantages thereof will be readily obtained, as the samebecomes better understood by reference to the following detaileddescription, when considered in connection with the accompanyingdrawings, wherein:

FIGS. 1A, 1B, and 1C show side and overhead views of fiducial implants.

FIGS. 2A and 2B show a side and overhead view of a preferred positioningscheme of fiducial implants in the skull.

FIG. 3 is an offset view of two coordinate systems that have undergonetranslation with respect to each other.

FIG. 4 is an offset view of two coordinate systems that have undergonerotation with respect to each other.

FIG. 5 and FIGS. 5a, 5b and 5c are offset views of two coordinatesystems that have undergone translation and rotation with respect toeach other.

FIG. 6 is a flow chart with respect to determining the same point P attwo different times in an internal coordinate system to the body.

FIG. 7 is a side view of a preferred embodiment of the presentinvention.

FIG. 8 is a flow chart with respect to determining the location of apoint P in an internal coordinate system with respect to an externalcoordinate system.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1A, 1B and 1C there is shown a fiducial implant 10 for the humanbody that is detectable by an imaging system. The fiducial implantcomprises a first portion 12 and a second portion 14. The first portion12 is configured to be detected by an imaging system (when placedbeneath the skin.) The second portion 14 is configured for fixedattachment to the bone beneath the skin without penetrating entirelythrough the bone and without fracturing the bone as will be described inmore detail later. The first portion 12 is of detectable size andcomprised of a material for detection by an imaging system andsufficiently small to provide minimal distortion of the skin when placedat an interface between the skin and the bone as will be described inmore detail later. First portion 12 also has at least a portion which isspherical and defines a surface for cooperating with a tool for securingthe second portion 14 to the bone. Additionally, the placement of threefiducial implants 10 into a portion of anatomy of the human body allowsfor the recreation of a particular image slice of the portion of theanatomy taken by an imaging system in order to duplicate images taken atthe first time period, that is, at the initial examination. Thisprovides a doctor with the ability to accurately follow the progress oftreatment on selected slices representing the anatomy of interest.

Moreover, the existence of three fiducial implants 10 allows a target (atumor for instance) to be identified relative to an external coordinatesystem. The portion of anatomy with the target may then be operated on,for instance, robotically, or precisely irradiated.

To allow for the accurate comparison of image slices from at least twodistinct periods of time, the three fiducial implants 10 are firstimplanted into a body of a patient at a desired region of interest. Thepatient is then placed in an imaging system and images of a series ofcross-sectional slices are obtained that include, for example, thevolume of the tumor which is the primary target of interest. From theimaging data obtained, the three fiducial implants are located and aninternal coordinate system is defined with respect to them. If it is sodesired, the image data may be further reformatted to show image sliceswhose direction is different from that obtained originally during theimaging period. Depending on the diagnostic information that these imageslices reveal, appropriate decisions with regard to surgery,chemotherapy or radiation therapy on a patient may be made. The imagingdata can also be used from several different types of images, such asCT, PET or NMR to obtain the same view of the anatomy but with differentqualities stressed.

If it is decided to obtain further imaging data at a later time, thenthe patient is returned to the imaging system and the procedure forobtaining image data is repeated. The fiducial implants 10 are locatedwith respect to the second imaging session and the same internalcoordinate system is defined relative to the implants 10. Once the sameinternal coordinate system is defined with respect to the second imagingsession, the translation and rotation of the internal coordinate systemand the images with it is determined with respect to the coordinatesystem established at the first imaging session. An image sliceidentified from the first imaging session that is to be used fordiagnosis, is recovered from the second imaging session. The two imageslices, one from the first image session and one from the second imagesession, are then compared to determine what changes, if any, haveoccurred in the anatomy of the patient.

More specifically, a 3-dimensional noncollinear coordinate systemrequires three distinct noncollinear points to be fully defined. Ifthere are more than three identifiable points, the system isover-determined and three points have to be chosen to define thecoordinate system. If there are less than three identifiable distinctpoints, the system is undetermined and a position relative to the one ortwo identifiable points will not be defined.

The known location of three distinct points identifies a plane uponwhich an orthogonal coordinate system can be established. If the threepoints are fixed in place relative to each other over time in the body,a coordinate system can be established that is also fixed in time. Theability to define a fixed internal coordinate system to the human bodyover time has important ramifications. A fully defined internalcoordinate system that is fixed in place over time with respect to somelocation in the body permits comparison of subsequent images of the bodytaken into imaging systems such as CT scans, NMR scans or PET scans, toname a few. More precisely, these comparisons will allow a diagnosticianto see what change, if any, has occurred within the body at apredetermined location.

By utilizing a fixed coordinate system relative to the body, the samecoordinates can be compared over time. However, the tissue or bodymaterial is not necessarily fixed in place relative to a predeterminedset of coordinates over time. After the passage of time, the tissue mayhave shifted, a change not uncommon following surgery. Nevertheless, theability to compare various properties (depending on the type of images)of the tissue at the same coordinates and at different times is a greatadvantage for diagnostic purposes.

In principle, the three points (that are necessary) to define acoordinate system can be chosen in a variety of ways. In one embodimentwith respect to the brain or head region, the two ears and a tooth, orthe two ears and the nose may comprise the three points. Alternatively,an image slice of the skull could provide a set of points from which thethree points would be chosen to create the coordinate system for thebody. Preferably, three fiducial points that are implanted into thebody, and create high contrast images during scanning, provide the mostreliable way to define a coordinate system. Ideally the three pointsshould be in the same approximate area of the body that is underanalysis, and also should be identifiable and measureable by differentimagery systems, such as CT imagers and NMR imagers.

To create a fully defined coordinate system the detection of threedistinct noncollinear fiducial points is required. With respect tocreating a fully-defined coordinate system anchored to the human body,the requirement of detection dictates the need that fiducial implants 10are made of a material that is detectable by a system imaging the humanbody. The fiducial implant 10 has a first portion 12 that provides meansfor marking a predetermined position within a body. See FIGS. 1A, 1B,and 1C. First portion, or marker 12, ideally provides a high contrast inan image compared to the surrounding material. The material marker 12 ismade of also provides as little distortion as possible to the image sothe appearance of artifacts is kept to a minimum. Marker 12 is also safefor use in the human body and is unobtrusive so no discomfort orself-consciousness is experienced by a wearer.

Marker 12 exhibits symmetrical integrity to facilitate its location bythe imaging system. When marker 12 is scanned, the symmetry insures thatany plane through the implant provides essentially the same image andthe ability to locate its center of mass. The importance of being ableto identify the center of the marker 12 lies in the fact that the sameexact point can be reproductibly found for use in defining thecoordinate system. Error is thus minimized from subsequent recreationsof the same coordinate system due to displacement of the coordinatesystem from a previous alignment. For instance, a sphere is the idealshape for a marker 12 with respect to symmetrical integrity since theimage of any plane of the sphere is always a circle.

By knowing the radius of the spherical object and applying standardalgorithms, the center can be determined of the spherical marker 12 fromany plane passing through the sphere. The algorithm for determining thecenter of a sphere may require operator interaction to mark theapproximate location of the implant. The center of mass can bedetermined with successful approximation from the boundary of thecircular profile identified through the operator's interaction. Forinstance, by having information about the density of the fiducialimplant's image and assuming it, then scan profiles through its imageresult in bell-shaped distributions, the boundary points of which can bedetermined therefrom. From the boundary points of the center of mass iscomputed. This may require additional slices depending on the size ofthe fiducial implant and its relative position with respect to adjacentslices, particularly when the physical size of the implant exceeds thatof the scan slice.

When the centers of mass of the 3 fiducials (10a, 10b, 10c) aredetermined, then two of them (10a, 10b) define for instance the x-axisvector of the coordinate system and the vector cross product of vectors10a, 10b and 10a, 10c fully determine the coordinate system as shown inFIG. 5a which is described more fully below.

Marker 12, which is 1 to 10 and preferably 4 millimeters in diameter,can be made of, for example, titanium in the form of a hollow sphere.The hollow of the sphere can be, for example, filled with agarose gelhaving various desired dopants, the choice of which depends on theimaging system used to best accent or highlight the marker 12. Marker 12is intimately connected to a second portion 14 of the fiducial implant10.

The second portion 14 provides means for anchoring the marker 12 intothe body. The site of preference for anchoring the marker 12 in the bodyis bone, since it provides a good material to hold the implant means inplace and also because bone stays in a fixed position over time in thebody. Anchor 14 is long enough to penetrate into the bone to which it isanchored, and long enough to be firmly embedded without fracturing thebone. Anchor 14 is 1 to 10 and preferably 3 millimeters long. Preferablythe anchor 14 should be screwed into the bone, rather than driven withan impact tool to lessen the chance of fracturing the bone. Anchor 14can also, for example, be made of titanium.

The fiducial implant 10 also has means 16 for receiving force so theanchor means 14 can be fixedly secured to the body. Where anchor means14 is a screw, preferably an indention 16 in the shape of a polygonrecess to receive an allen wrench is located in marker 12 (see FIG. 1c).The use of an allen wrench with the associated polygonal recess has moresymmetrical integrity than the cross shaped receptor site for a phillipsscrew driver or a single groove receptor site for a standard screwdriver.

The implantation of a fiducial implant 10 having an anchor 14, in thiscase a screw, preferably utilizes a trocar not shown, to penetrate theskin and reach a desired bone site. The trocar is first placed on theskin over the desired anchoring site and a piercing rod therein isforced through the skin. The piercing rod within the trocar is thenremoved while the trocar is kept in place. A rod with an allen wrenchhead fitted to the polygonal indentation 16 in the marker 12 of theimplant 10 is inserted into the trocar until the screw 14 portion of theimplant 10 contacts the anchoring site, for instance bone. Force is thenapplied to the portion of the rod extending out the trocar until theimplant 10 is embedded into the bone. Such a procedure is accomplishedunder local anesthesia and should only be about 5 minutes in length.

The placement of the three fiducial implants 10 depends on the portionof the anatomy to be evaluated. Essentially, three fiducial implants 10are placed in three locations such that they are readily identifiableand the locations are fixed with respect to each other over time. If,for example, a study of the skull and brain is to be undertaken,preferably an implant 10A is placed on the midline of the skull 18 justabove the hairline, with the other two implants 10B, 10C being placed onthe right and left side, respectively, of the midline in a posteriorposition to the midline implant 10A. See FIGS. 2a and 2b which are afrontal and overhead view of the skull 18, respectively. Another exampleof an area of interest could be the torso, with one fiducial implant 10placed on the midline of the sternum and the other two fiducial implants10 placed laterally thereto on the right and left side, respectively,and in a rib. Or, one fiducial implant 10 can be placed in the spinousprocess of a vertebra in the midline and the other two fiducial implantsplaced in the right and left illiac crest, respectively.

Imaging apparatus provides a fixed axis relative to which any otherposition in space can be located. As a result, the position of thefiducial marker and the coordinate system these markers define can belocated relative to the imaging apparatus. The features of the inventionpermit the location of the markers relative to the imaging apparatus tobe recorded for future reference. In subsequent scans, the patient'sorientation may change relative to the imaging apparatus. This neworientation can be measured by locating the fiducial markers in relationto the image apparatus and comparing it to the previously recordedlocation. The comparison technique permits re-orienting images ofsubsequent scans to a position corresponding to the earlier recordedscan so that image slices are always at generally the same cross-sectionof the earlier recorded slices.

In actual operation, these positions are defined by the coordinatesystem and it is the position of these systems that is accomplishedtranslation and rotation as discussed below.

Once the fiducial implants 10 are in place and a coordinate systemdefined, subsequent images of the same anatomical volume area can becompared. FIGS. 3, 4 and 5 show blocks that represents a person's headfor purposes of illustration. If, for example, images of the brain arebeing taken, a person's head may be placed below, above or to the side(see FIG. 3), of its location at a previous imaging session. The headmight be rotated (see FIG. 4), as compared to its orientation during anearlier imaging session. The head might have undergone rotation andtranslation as compared to a previous imaging session, see FIG. 5.Regardless of the reason why the head is oriented differently, by takingadvantage of the fixed fully-defined internal coordinate system in thebrain a previous point or slice image of the brain can be obtained fromsubsequent image information. This is accomplished as shown in FIG. 6,by comparing the location and direction of the plane defined by thethree fiducial points at the first examination with the location anddirection of the same plane defined by the three fiducial points at thetime of the second examination. For simplicity, the origin of thecoordinate system is located at a given fiducial point. By measuring thedistance in say, the x, y and z directions between the same fiducialpoint (the origins) at the two different times, the translation of theorigin of one coordinate system with respect to the other can beobtained.

Preferably, one can carry out the transformation with respect torotation from a given cartesian coordinate system to another by means ofthree successive rotations performed in a specific sequence. Threeangles known as the Eulerian angles are then defined. These threeEulerian angles are the three successful angles of rotation that arerequired to carry out the transformation. The determination of theEulerian angles is accomplished by first computing the intersection oftwo planes determined by the fiducial implants, then computing the anglebetween the fiducial x-axis and the line of intersection (psi), thencomputing the angle theta; and then computing the angle phi. At thispoint the three Eulerian angles are determined. For the example given inFIGS. 5a, 5b and 5c the sequence that is required to carry out thetransformation is started by rotating about the z axis as shown in FIG.5a (For other transformations, a sequences of translations and rotationscan be used.) The resultant coordinate system is labelled thexi,eta,zeta axes . In the second stage, the intermediate axes,xi,eta,zeta, are rotated about the xi axis counterclockwise by an angletheta to produce another intermediate set, the xi',eta',zeta' axes asshown in FIG. 5b where the third fiducial implant 10c is not shown tosimplify understanding. The xi' axis is at the intersection of the xyand xi'eta' planes and is known as the line of nodes. Finally thexi',eta',zeta' axes are rotated counterclockwise by an angle to producethe desired x'y'z' system of axes as shown in FIG. 5c. The Eulerianangles theta, phi and psi thus completely specify the orientation of thex'y'z' coordinate system relative to the xyz coordinate system and cantherefore act as the three needed generalized coordinates.

The elements of the complete transformation A can be obtained by writinga complete transformation matrix as the triple product of the separaterotations, each of which can be written in matrix form. Thus the initialrotation about the z axis can be described by the matrix D:

    xi=Dx

where xi and x stand for column matrices. Similarly, the transformationfrom xi,eta, zeta, to xi',eta',zeta' can be described by the matrix C:

    xi"=Cxi

and the last rotation to x'y'z' by a matrix B

    x'=Bxi'

Thus the matrix of the complete transformation can be written as

    x'=Ax

which is the product of the successive matrices:

    A=BCD

The matrix D can be written as ##EQU1## The matrix C can be written as##EQU2## The matrix B can be written as ##EQU3## The product matrixA=BCD is then obtained with the help of the above expression. The orderof the matrix multiplication depends upon the task identified; in thepresent case it defines the transformation from the xyz set of axes tothe x'y'z' set of axes.

Once the Euler angles are determined, the problem of orientation issolved, at least, in principle. A major simplification of thecomputation can, however, be achieved if Euler's theorem is implemented.

Euler's theorem on the motion of a rigid body states: that the generaldisplacement of a rigid body with one point fixed is a rotation aboutsome axis.

If the fixed point is taken as the origin of the body set of axes, thenthe displacement of the rigid body involves no translation of the bodyset of axes, the only change is the orientation. The theorem then statesthat the body set of axes can always be obtained as a single rotation ofthe initial coordinate system. It is characteristic of rotation that itleaves the direction of rotation unaffected by the operation. In otherwords, any vector lying in the direction of the axis of rotation musthave the same components before and after the rotation. A necessarycondition is that the magnitude of the vector should be unaffected andis automatically provided by the orthogonality conditions. Thus Euler'stheorem can be proven if it is shown that there exits a vector R havingthe same component before and after the transformation, that is, in bothsystems. From this it follows, that

    R'=AR=R

The above is an eigenvalue problem that can be written as

    AR-kR=O

where k is constant. The values for which k is soluble are calledeigenvalues of the matrix.

The eigenvalue equations may be written

    (A-kl)R=R

This equation comprises a set of three homogeneous simultaneousequations for the components X,Y,Z of the vector R. Because of this theycan never provide the definite values of the three components, onlytheir ratios. Thus the magnitudes of the components remain undetermined.For homogeneous equations the determinant of the above equation has tovanish, and the solution provides the values of k. For the real,orthogonal, matrices the equation must have k=+1.

In general the equation has three roots corresponding to threeeigenvectors. The consideration lead to diagonal matrix of k ##EQU4##The matrix equation can then be written

    AR=Rk

or multiplying from the left by R**(-1)

    R **(-1)AR=k

This equation provides a useful approach to the problem: seek a matrixthat transforms A into a diagonal matrix, the elements of which are thedesired eigenvalues.

Finally the angle of rotation has to be determined. The directioncosines of the axis of rotation can be obtained by setting k=1 in theeigenvalue equation and solving for the components of R. It can be shownthat the trace of the matrix A can be used to determine the angle ofrotation w. One has to compute the trace of A, i.e. T, that is,

    T=1+cos W

from which W can be determined.

For the rotations described above to have any meaning, the fiducialimplant 10A, or some point, must be at the same place for the twocoordinate systems that are being aligned. This requires a translationof the fiducial implant 10A at a location corresponding to onecoordinate system into the location of fiducial implant 10A at the othercoordinate system. By simply moving the desired coordinate system thelinear amounts of x, y and z, with respect to a cartesion coordinatesystem, the fiducial implant 10A is situated at the same location. For amore complete discussion of the transformation of a cartesian coordinatesystem into another, see Herbert Goldstein, Classical Mechanics, AddisonWesley, Reading, Mass., 1965, pp. 107-109.

Thus, any point can be obtained with respect to translation and rotationof a given cartesian coordinate system. Since any point can be obtained,any plane can also be obtained, because a plane is comprised of a set ofpoints. For example, if a given point is desired to be looked at overtime, then the coordinate of the point is identified with respect to afirst time. The translation and rotation information corresponding tothe coordinate system at the first time with respect to the second timeis then applied to the point at the first time to indicate thecoordinates of the identical point in the coordinate system at thesecond time. The imaging data pertaining to the second time is thensearched to find the desired point. This is but one way of many possibleways to obtain the same point in the coordinate system as a function oftime.

Similarly, for a plane or slice image, the same procedure is applied toeach point of the set of points that make up the slice image. Thedesired points are then searched for in the image informationcorresponding to the coordinate system at the second time. Once all thepoints, with their associated image information are identified, they arereformatted to produce an image slice as close as possible to thedesired image slice pertaining to the coordinate system at the firsttime. Of course, the position of the slice selected by the physicianfrom the initial image slices has to be determined with respect to thefiducial implants. To this end, preferably, the z coordinates or theelevation coordinates of the system have to be introduced. This can bedone with respect to any slice in the image set. For instance, the slicecontaining the first fiducial implant can be chosen.

Ideally, the reformatting step takes image points from image slices ofthe second time and aligns them together and produces an image slice assimilar as possible to the desired image slice of the first time. Inpractice, however, quite often a point that is necessary for thecreation of a reformatted image does not exist because image slices weretaken for instance above and below the point. In this case interpolationmust be used to estimate the attributes of the missing point so adesired image slice can be prepared. For example, one simple method ofinterpolation utilizes the two closest known points to the nonexistentdesired point. These two known points are also as nearly opposite eachother as possible with the desired point therebetween, and averagestheir image value. For example, if the intensity of the image associatedwith one point is 6 units on a scale of 1 to 10 units and that of thesecond point is 4 units, and the two points are essentially equal indistance from the desired point, the desired point is assigned an imageintensity value of 5 units. See FIG. 6 which shows the flow chartdescribing the above overall process.

Interpolation could be avoided if the internal coordinate system ispositioned identically at the different times the imaging data isobtained. This could be accomplished by causing the three fiducialimplants 10 to be at exactly the same position whenever imaging data isobtained. By having, for instance, an X-ray machine, or following themethod discussed below that reveals the location of the fiducialimplants in the body with respect to an external coordinate system, andknowing where the implants were positioned at the first time thatimaging occurred, the body could be moved to be in the same exactlocation. One way of moving the body in position is with a table orplatform that has 3 dimensional movement. Then, knowing where thecoordinate system is in the body with respect to the platform, theplatform could be moved up, down, forward, backward and/or rotated sothe internal coordinate system is positioned exactly the same way it wasthe first time imaging data was obtained.

To summarize, and referring to FIG. 6, the procedure consists of thefollowing steps:

1. Locating the fiducial implants in the initial examination image set,and establishing the internal coordinate system;

2. Selection of the slice(s) of interest in the initial set;

3. Determination of the translation distance between the coordinatesystem determined by the fiducial implants and the selected slice;

4. Localization of the fiducial implants in the follow-up study;

5. Determination of Eulerian angles in the coordinate system;

6. Determination of the coordinates of each point in the transformedslice corresponding to the selected slice in the initial system;

7. Determination of the intensity values at each point usinginterpolation in the axial direction. (Axial direction is defined as thedirection of motion of the imager table).

Although there are many different hardware and software embodiments toimplement processing of the image data, each can be divided according toits functioning as follows:

(1) hardware that facilitates fast reconstruction of the cross sectionalimage;

(2) operator-interactive image display;

(3) storage device for images;

(4) hardcopy capability for images.

One embodiment utilizes the existing computer and its peripherals togenerate the reformatted images.

Another embodiment utilizes a stand-alone system, in which the imagesare fed from the respective imager, and then perform the comparativeanalysis in the stand-alone system. The whole computer part of theimager must be essentially duplicated, plus various options for datainput supplied, in order to accommodate images of all types. Hardcopycapability is also desirable therein, such as a matrix camera, becausepermanent records are invaluable to the diagnostician.

Whether a stand-alone system or an existing system is modified forimplementation of the above described reformatting, the images arepreferably stored as files having two parts: (1) the header thatcontains the patient's demographic data and information on theexamination itself, that is, technical parameters of the exposure orimage procedure; and (2) the image matrix. These two parts arepreferably stored temporarily (for a couple of days, usually) onmagnetic disk drives, and then moved to permanent storage medium, suchas magnetic tape or floppy disk. In addition to this file structure asubfile may be added containing the results of the computation (theEuler angles may be added, for instance).

An apparatus 100 carries out the imaging, signal processing and displaynecessary to provide images of essentially the same coordinates in thehuman body which can be compared over time, or to provide the locationof targets, such as tumors is shown in FIG. 7. Such an apparatus 100 iscomprised of an imager 102 that supplies imaging data and is controlledby a programmable computer 104. The imaging data is obtained from asource 106 in the imager 102 that is approximately placed about apatient 107 as is well known in the art. The imaging data experiencessignal processing, as described above, and the desired images aredisplayed on display 108. Additionally, operator interaction can beachieved through an operator control panel 110 and the coordinates of atarget can be displayed in the coordinates of the target display 112 forradiation therapy applications.

An application that takes advantage of a fully-defined internalcoordinate system of the body relates to radiation therapy. Forradiation therapy the location of a radioactive beam of an externalcoordinate system must be related to the internal coordinate system. SeeFIG. 5 where the external coordinate system can be considered theunprimed system and the internal system the primed system. The point Pcan represent the location of a point of a tumor. In this situation theactual distances and locations of the point P in the primed coordinatesystem, and the location of the origin s of the primed coordinate systemare important. If the point P is known with respect to the internal orprimed coordinate system, and the primed coordinate system is known withrespect to the external or unprimed coordinate system and the Eulerangles of rotation are known, then the location of point P is known withrespect to the external coordinate system. For example and referring toFIG. 7, in radiation therapy or surgery knowing where the internalcoordinate system A is with respect to an external coordinate system Bhas many uses. In radiation therapy if the location of a tumor is knownwith respect to the internal coordinate system and the internalcoordinate system is known with respect to an external coordinate systemhaving a radiation source 20, such as an x-ray machine for killingcancer cells, then radiation can be applied only to the tumor providedit can concentrate on the volume of the tumor only. This would removethe guess work of a radiotherapist looking at various images of a tumorin a body and estimating where to aim the radiation source so,hopefully, only the tumor is irradiated. The location of a tumor in aninternal coordinate system can be identified for instance, by a firstimaging session. The data therefrom is stored in a medium that allowsits recall when the tumor position is desired to be known and it is notdesired to have to retake images of the anatomy.

One way to accomplish the irradiation of a specific location in the body32, where, for instance, a tumor is located, involves the use of a robotarm 34 whose base 36 can be chosen as the origin (0,0,0) of the externalcoordinate system B. At the tip 38 of the robot arm 34 is located asensor 40. The sensor 40 can be a metal detector or an ultrasonicdetector or any instrument that can sense the position of a fiducialimplant 10 in a body 32. If the fiducial implants 10 are placed in askull 18 and there is a tumor therein, the sensor 40 in the tip 38 ofthe robot arm 34 is moved by the arm 34 until it contacts a fiducialimplant 10 in the skull 18. The movement of the robot arm 34 is trackedby a computer (not shown) so the position of the sensor 40 relative tothe arm's 34 base 36, the origin 0 of the external coordinate B, isknown. The means to track the arm is well known and is accomplished bysensors (not shown) in critical locations of the arm 34, detectingrotation or movement of the joints 42 of the arm 74. By supplying thisinformation to a computer along with the information of the fixedlengths of the structure of the robot arm 34, the tip 38 location of thearm 34 is always known. When the tip 38 of the arm 34 rests on thefiducial implant 10 in the skull 18, the location of the internalcoordinate system A defined by the fiducial implants 10 is known withrespect to the external coordinate system B. Supplying the Euler anglesof rotation and the location of the tumor which is known relative to theinternal coordinate system A to the computer, provides the ability todetermine the location of the tumor in the external coordinate system B.The location of the tumor is known relative to the internal coordinatesystem through for instance the image data already stored, and the factthat the fiducial implants 10 are also fixed relative to each other oncethey are in place. The radiation source 20 and where it is aimed isknown by the computer relative to the external coordinate system B. Thecomputer, having the information where the tumor is located in theexternal coordinate system B, can aim the radiation source 20 toprecisely irradiate the tumor site in the brain. In general, thelocation of a point P in the internal coordinate system relative to theexternal coordinate system is determined when the distance between theorigins of the two coordinate systems is known and the Euler angles areknown, as described above.

In surgery, the internal coordinate system defined by the three fiducialpoints can allow, for example, a laser to be followed as it cuts throughtissue to a tumor. An imaging system present in the operating theaterwould be positioned to continually take imaging data that is provided toa computer system which also guides the laser based on the inputteddata. As the laser cuts through the tissue, the change in the tissue isapparent through the imaging system and can be followed with respect tothe fixed internal coordinate system. When a predetermined position isreached by the laser, or a predetermined portion of tissue has beenremoved by the laser, the computer controlling the laser and processingthe imaging data would discontinue the operation of the laser.

In the operation of the invention, after the fiducial implants are inplace in a patient, imaging data is taken at a first time and stored. Atdistinct intervals in time, for instance about every year thereafter,the patient returns to the location of the imaging system or one similarto it, and undergoes follow-up imaging. The most recently receivedimaging data is then reformatted, as described above, to obtain highfidelity images of the same cross-sections on the body as attained inthe earlier session. The images from the latest session are thencompared with the earlier session (if there are many earlier sessionsthey can all be used for comparison purposes) to determine if there havebeen any significant changes such as progression or regression of anabnormality, such as a tumor. The imaging data collected from variousimaging sessions taken at different time intervals can, of course, becompared many ways such as by reformatting images taken at earliersessions to show an image slice of interest chosen from the latestsession, instead of just comparing image slices of a latest session tothose of an earlier session. The purpose of the comparisons, as statedearlier can be multifold: (a) either a simple follow-up of the growth ofthe tumor, without therapy; or (b) verification of therapeutictreatment, such as radiation or chemotherapy or (c) follow-up ofsurgical treatment.

In the operation of the invention with regard to radiation therapy, thetumor is first identified in the patient's body. The patient is thenpositioned in the imaging system such that at least the tumor area canbe imaged. The imaging system is used to locate the position of thetumor in the internal coordinate system. The image data can, forinstance, then be stored for later use so the tumor position isidentified without new images having to be obtained every time radiationtherapy is performed. The patient can then be placed before a radiationsource, and each time radiation therapy occurs, the information from theimaging session that is stored is supplied to the computer operating theradiation source. The internal coordinate is located with respect to theexternal coordinate system, for instance by locating one fiducialimplant, as described above, with respect to a known position in theexternal coordinate system. Once the position of the internal coordinatesystem is known with respect to the external coordinate system, thetumor position is known with respect to the external coordinate system,since the tumor position is already known with respect to the internalcoordinate system from the stored imaging information. A radiationsource is then aimed, for example by a computer receiving the imagingand position data, at the tumor in the body. With respect to surgery,the procedure that is followed to take advantage of the fiducialimplants is similar to the procedure described above for radiationtherapy. Once the tumor is located with respect to the internalcoordinate system, and the location of the internal coordinate system isknown with respect to the external coordinate system, the tumor islocated with respect to the external coordinate system. Surgicalinstruments can then be guided to the tumor by the computer with theimaging system placed in an interactive mode therewith. The imaging datathat the imaging system constantly feeds the computer allows thecomputer to track the progress and the extent of the surgery.

Obviously, numerous (additional) modifications and variations of thepresent invention are possible in light of the above teachings. It istherefore to be understood that within the scope of the appended claims,the invention may be practiced otherwise than as specifically describedherein.

What is claimed is:
 1. An apparatus for locating a target on a portionof a patient comprising:a. at least three fiducial implants; b. imagingmeans for producing sliced images of a desired cross-section of theportion of the patient's anatomy; c. a display means for displaying theimages produced by said imaging means; d. a programmable data-processingcomputer; e. means for selecting from said at least three fiducialimplants three fiducial implants for defining an internal coordinatesystem with respect to the human anatomy identifiable by said imagingmeans, said means being hidden from the exterior of the anatomy butdetectable by said imaging means; f. said computer being programmed tostore images by said imaging means during a first scan; g. said imagingmeans and said computer cooperating to produce sliced imagessubstantially identical to images of said first scan; h. means fordefining an external coordinate system using said fiducial implants; i.means for relating said internal coordinate system with respect to saidexternal coordinate system whereby the target can be located.
 2. Theapparatus according to claim 1 further comprising a robot arm, saidrobot arm having positions known by the computer with respect to theexternal coordinate system, and said computer being programmed to definethe relationship between the internal coordinate system and the externalcoordinate system.
 3. The apparatus of claim 2 wherein said robot armincludes an arm and means for moving said arm to a position adjacent atleast three of said fiducial implants to define said external coordinatesystem.
 4. The apparatus according to claim 3 further comprising meansfor destroying the target.
 5. The apparatus according to claim 4 whereinsaid means for destroying the target includes a laser.
 6. The apparatusaccording to claim 4 wherein said means for destroying the target is aradiation means for directing radiation to the target as defined by thecomputer.
 7. The apparatus according to claim 4 wherein said means fordestroying the target includes computer means for directing the targetdestroying means to the portion of the target to be destroyed based onsaid internal coordinate system.